Method and apparatus for the removal of polyvalent cations from mono ethylene glycol

ABSTRACT

The invention relates to a method and an apparatus for the removal of polyvalent cations, in particular divalent cations, from mono ethylene glycol. Mono Ethylene Glycol (MEG) is used to prevent hydrate formation in pipelines transporting gas, condensate and water. It may also contribute to pipeline corrosion control. The invention describes a method for the removal of polyvalent cations from mono ethylene glycol, comprising providing a feed of aqueous mono ethylene glycol comprising dissolved gas and salts of divalent cations (rich MEG), heating the aqueous mono ethylene glycol to a heated mixture, causing precipitation of at least part of the salts and release of at least part of the dissolved gas, in particular carbon dioxide, separation of released gas from the mono ethylene glycol, separation of at least part of the precipitated salts from the mono ethylene glycol, distillation of at least part of the water from the heated mixture, to yield hot dewatered mono ethylene glycol (lean MEG), wherein a first part of the hot dewatered mono ethylene glycol is lead back to the aqueous mono ethylene glycol feed to provide at least part of the heat for heating the aqueous mono ethylene glycol.

FIELD OF THE INVENTION

The invention relates to a method and an apparatus for the removal ofpolyvalent cations, in particular divalent cations, from mono ethyleneglycol.

BACKGROUND OF THE INVENTION

Mono Ethylene Glycol (MEG) is used to prevent hydrate formation inpipelines transporting gas, condensate and water. It may also contributeto pipeline corrosion control. Typically, a 90 wt % MEG solution isinjected to the gas stream at the beginning of the pipeline. A waterphase may or may not be present at the pipeline inlet. As the streamcools down though the pipeline, water will condense from the gas. Waterand MEG will mix completely and the mixture of aqueous methyl ethyleneglycol is called rich MEG, because it is rich in water. The MEGconcentration in the rich MEG can be 30-80 wt %, typically 50-65 wt %.

The rich MEG is separated from the gas and the condensate in one orseveral separation stages. Normally the rich MEG is heated to 30-80° C.to improve separation and avoid formation of emulsions/foam. Filters,centrifuges, decanters, coaleshers etc. may be included to improve theseparation process and for removal or particles. The rich MEG is thennormally sent to a storage/buffer tank, before it is sent to aregeneration unit to remove impurities. The processing of rich MEG iscalled pre treatment.

MEG is regenerated in a regeneration unit which typically consist of areboiler/heater and a distillation column. The rich MEG is heated in thereboiler and most of the water is evaporated to produce the desired MEGconcentration in the reboiler, normally around 90 wt %. The feedingpoint may be either directly into the reboiler or in the distillationcolumn. The vapour is distilled to remove MEG and produce water with aslow MEG concentration as possible, typically below 1000 ppm.

The rich MEG may contain many contaminants, such as ions, particles andvarious production chemicals. Especially troublesome are divalentcations such as iron, calcium, barium, strontium and magnesium becausethey can precipitate as various carbonates and hydroxide salts in theregeneration system. Carbonate salts from divalent cations tend toprecipitate on hot surfaces because their solubility decreases withincreasing temperature. To avoid scale problems in the reboiler, theseions should be removed from the rich MEG before entering the reboiler orother distillation equipment.

A solution for chemically removing certain salts from MEG is describedin WO 2009/017971, which describes a reclamation unit that removes thesalt as a pre-treatment before distillation, by adding chemicals such asNaOH, NaHCO3, Na2CO3 to increase pH and carbonate concentration. Thiswill lead to high super saturation of the carbonate salts of thedivalent cations and they will thus precipitate and can be filtered off.

If chemicals are added, they will accumulate in the MEG unless they aretaken out later. Adding NaOH, NaHCO3, Na2CO3 or similar to increase pHmeans that sodium concentration increases and also the alkalinity. (Itis also possible to use the similar potassium salts). A reclaimer stagemay have to be included to control the salt concentration level in theloop. Having to use the reclaimer makes the process more complicated inaddition to increased chemical costs and relatively high energyconsumption.

Another solution is to precipitate polyvalent cations by preheating theMEG. This can be done by a preheater unit positioned before thedistillation unit. Heating the rich MEG will lead to salt precipitation,which may subsequently be removed by some solids removal process such asfiltering, centrifuge or settling. Heating alone may, however, beinsufficient to get quantitative precipitation of the divalent cationsand the reaction rate can be slow, requiring a large flash drum orsimilar tank to increase the retention time.

OBJECT AND SUMMARY OF THE INVENTION

It is an object of the invention to provide a method for removingpolyvalent cations from aqueous MEG that is relatively simple. It isanother object of the invention to provide a method to remove polyvalentcations from aqueous MEG that has a relatively low energy consumptionand without additions of any chemicals.

The invention provides a method for the removal of polyvalent cations,in particular divalent cations, from mono ethylene glycol, comprisingthe steps of:

-   -   a) providing a feed of aqueous mono ethylene glycol (rich MEG)        comprising dissolved gas and salts of divalent cations,    -   b) heating the aqueous mono ethylene glycol to a heated mixture,        causing precipitation of at least part of the salts and release        of at least part of the dissolved gas, in particular carbon        dioxide,    -   c) separation of released gas from the mono ethylene glycol,    -   d) separation of at least part of the precipitated salts from        the mono ethylene glycol,    -   e) distillation of at least part of the water from the heated        mixture, to yield hot dewatered mono ethylene glycol (lean MEG),        wherein a first part of the hot dewatered mono ethylene glycol        is lead back to the aqueous mono ethylene glycol supplied in        step a) to provide at least part of the heat for heating the        aqueous mono ethylene glycol in step b).

The feed of aqueous MEG, also called rich MEG, can have various sources,but is typically derived from a gas transport pipeline. The rich MEGstream typically comprises a MEG concentration of 30-80 wt %, preferably50-65 wt %. The temperature is typically from 5-100° C., preferably30-60° C. The pressure may vary from atmospherical pressure to 50 bar,typically in the range of 1-5 bar. The multivalent cations present inthe rich MEG stream may comprise various elements, for instance Ca, Mg,Ba, Sr. Iron cations, which may exist in various oxidation states, areparticularly troublesome if they would precipitate on the surface ofequipment, and may be present in concentrations of for instance 0-150ppm, typically 5-30 ppm. The solution will typically also comprisemonovalent cations, in particular sodium. The dissolved gases wouldtypically include carbon dioxide as a main component. Sodium and carbondioxide are typically present in the form of the corresponding salts, inparticular NaHCO₃.

Heating of the rich MEG induces precipitation of the polyvalent cations.Once precipitated as salts, the obtained particles may be removed by forinstance filtration at a later stage in the process. The solid particlesare less prone to settle on equipment than the dissolved cations. Thus,when equipment such as a reboiler is to be protected, most of theprecipitation should take place before the mixture is introduced intothe reboiler .

The removal of gas, in particular carbon dioxide, assists inprecipitating salts. Carbon dioxide is an acid when dissolved withwater, and removal will raise the pH of the mixture. The gas removal,for instance in a flash separator under reduced pressure, will furtherenhance the reaction rate of precipitation. The combination of gasseparation and heating will lead to a useful precipitation rate.

The separation of at least part of the precipitated salts from the monoethylene glycol may be done at one or more places in the process stream,although it is preferred if this is done at the output stream of MEGbefore further transport or storage.

During distillation of the rich MEG, the temperature is further raisedand water is mostly removed, in order to obtain dewatered MEG, alsocalled Lean MEG. In lean MEG, the concentration of MEG is raised due tothe removal of water, having concentrations of 70-99 wt %, preferably85-95 wt %. The temperature of the liquid in the distillation phase,typically using a reboiler, is usually in the range of 100-160° C.,preferably 120-150° C. The pressure is typically in the range of 1-2bar. As the distillation will remove water as well as remaining carbondioxide, the pH will be higher than the rich MEG, typically in the rangeof pH 9-14, preferably pH 10-12. At the distillation stage, the mixturewould contain precipitated salts. As most of the precipitation wasalready induced before entering the distillation, by pre-heating andpreliminary removal of carbon dioxide, the precipitation on the warmsurfaces of the distillation equipment, in particular the reboiler,would be significantly reduced compared to direct introduction of richMEG. Part of the dewatered (lean) MEG can be collected for storage,after filtering or otherwise separating the precipitated saltscontaining polyvalent cations. In the method according to the invention,at least part of the relatively hot lean MEG will be reintroduced intothe feed of aqueous (rich) MEG. This provides a means of heating theaqueous MEG to induce new precipitation. This also allows to perform thepreheating without a dedicated preheater, or with a preheater that has alower capacity, making the process and the used equipment simpler,easier and more compact. By monitoring the temperatures of the lean MEGand rich MEG, the amount of hot lean MEG lead back to the feed line toachieve the desired preheating temperature can be determined andcontrolled.

It is preferred if in step b), the aqueous mono ethylene glycol isheated to at least 60° C., preferably from 60° C. 120° C., mostpreferably approximately 80-100° C. At such temperatures, precipitationand release of carbon dioxide is induced at an efficient rate.

Preferably, the hot dewatered monoethylene glycol lead back to the feedhas a temperature above 100° C., preferably above 120° C., mostpreferably approximately 145° C. Depending on the starting temperatureof the rich MEG feed typically in the range of 30-60° C., this allows tomix in a relatively low amount of lean MEG into the rich MEG feed. It ispreferred if the volume ratio of lean MEG to rich MEG is from 1:5 to5:1. Preferably, the mixture of rich MEG and lean MEG would comprisefrom 30-80 volume % lean MEG, most preferably around 50-70 volume % leanMEG. The temperature of the lean MEG is measured at the position wherethe split off part leaves the distillation unit. Preferably, the amountof hot dewatered monoethylene glycol lead back to the feed is controlledto obtain a warm aqueous monethylene glycol mixture in the temperaturerange of 60° C. 120° C. in step b), for instance by using control valvesor mixing equipment.

In a preferred embodiment the hot dewatered monoethylene glycol (leanMEG) lead back to the feed comprises particles of salts of divalentcations. The particles already present in the returned lean MEG act asnucleation sites and further enhance the rate of precipitation afterheating. Also, this prevents precipitation on equipment as theprecipitation rate on a preformed particle is usually faster thanprecipitation on the surface of equipment.

If the separation of at least part of the precipitated salts from themono ethylene glycol d) is performed on a second part the hot dewateredmono ethylene glycol that is not lead back to the feed. Having particlesin the returned hot lean MEG stream may be achieved by separatingparticles the MEG after the destillation, or only partial separation, orby only separating particles from the MEG that is taken out of theprocess for further transport or storage. This uses the separation unitssuch as filters more efficiently, and simplifies the control of theprocess significantly.

It is preferred if in distillation step e) the dewatered monoethyleneglycol is circulated in a distillation loop comprising at least oneheater, wherein a first part of the dewatered monoethylene glycol leadback to the feed is taken from the circulating monoethylene glycol. Manydistillation units comprise a circulation loop, and taking the dewateredMEG from the circulation loop makes it easier to provide a stable andmore homogenous stream of hot lean MEG back to the feed, making theprocess more controllable. Most preferably, the dewatered monoethyleneglycol is taken from the distillation loop after distillation and beforethe heater.

Most preferably, the distillation loop comprises a circulation pump,wherein the dewatered monoethylene glycol is taken from the loop afterthe recirculation pump and before the heater. This allows the pressurefrom the recirculation pump to be used for returning the lean MEG to thefeed without need for a separate pump.

It is preferred if the salts of polyvalent cations in the monoethyleneglycol comprises salts derived from at least one of the elementsselected from the group consisting of iron, calcium, barium, strontiumand magnesium, or mixtures thereof. The precipitates salts are typicallya mixture of carbonate salts, hydroxides and oxides. In a preferredembodiment, the salts comprise iron salts. Iron salts are mostpreferably removed from MEG, as these are among the most difficultspecies to remove from the surface of equipment once precipitated there.

The invention further provides an apparatus for the removal ofpolyvalent cations from mono ethylene glycol comprising a feed line fora feed of aqueous mono ethylene glycol, connected to a flash separator,for separating released gas and precipitating salts from aqueous monoethylene glycol, wherein the flash separator is coupled to - a reboilerprovided with a distillation unit for removing water from the mixturesupplied therein by heating, having an outlet to supply at leastpartially cleaned mono ethylene glycol, - wherein the outlet is providedwith a loop back to supply at least part of the hot cleaned monoethylene glycol to the supply line for aqueous mono ethylene glycol,adapted for heating the feed in the feed line by mixing, wherein theloop back is connected to the feed line upstream from the flashseparator. This equipment is adapted for performing the method asdescribed herein. As described above, the reboiler for distillation maycomprise a recirculation loop with a recirculation pump, wherein therecirculation pump is positioned to also provide pressure for the loopback.

In a preferred embodiment, the feed line is provided with a preheater,wherein the loop back is introduced into the feed after the preheater.The preheater makes it easier to control the temperature forprecipitation when combined with the heat provided by the recirculatedlean MEG. Having a preheater also makes it easier and faster to start upthe process, for instance after maintenance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 describes an example of a process and apparatus for removingpolyvalent cations from monoethylene glycol.

DESCRIPTION OF PREFERRED EMBODIMENTS

An example of the process and system is described in detail in FIG. 1.The rich MEG (1) containing the ions is mixed with a hot lean MEG stream(17). The mixed stream (2) should have a temperature in the range60-120° C., typically 80° C. before it enters a flash separator (3)where the precipitation will occur. Due to the heating, at least part ofthe dissolved gas is released and is vented off (4).

The hot mixture (5) is then fed to the reboiler unit (6). The feed pointis preferably located directly into the reboiler, but may also belocated in the recirculation stream (9, 11, 12, 14), preferablydownstream from the heater. The liquid phase in the reboiler (6) iscirculated (9) by a circulation pump (10) via (11, 12) to a heater (13)where the circulating liquid is heated before it is lead back to thereboiler (6). Water, MEG and other volatile components will evaporateand go to a distillation tower (7) where MEG is condensed and drainedback to the reboiler (6). The water vapour leaving the top (8) of thedistillation column is condensed in a standard reflux system. Part ofthe condensed water is used for reflux in the column (not shown in thefigure).

In the regeneration process, the rich MEG is boiled to evaporate waterto produce typically 90 wt % MEG, called lean MEG. Since water isevaporated, there is less solvent to dissolve ions and they areconcentrated. The alkalinity of the mixture will therefore increase,leading to increase in pH. Another important factor is that duringboiling, remaining CO₂ and other gasses are stripped off the mixture.Rich MEG typically has a pH of 5-8, meaning that most of the dissolvedCO₂ is in the form of bicarbonate (HCO₃ ⁻). When CO₂ is stripped off,the bicarbonate is converted to carbonate (CO₃ ²⁻) and this furtherincreases the pH:

2HCO₃ ⁻→CO₂(g)+CO₃ ²⁻+H₂O

The lean MEG solution thus has higher MEG concentration, higheralkalinity, more carbonate and higher pH. All these are factors thatwill promote precipitation of carbonate salt of the divalent/polyvalentcations.

In the present invention, some of the hot lean MEG (typically 145° C.)is recirculated and mixed with the cold rich MEG to obtain a temperaturein the range 60-120° C., typically about 80° C. It is also possible touse a combination of inline heating and recirculation to both obtain theoptimum recirculation rate and the optimum temperature. Due to theincreased temperature, MEG concentration, alkalinity, carbonateconcentration and pH, a substantial percentage of the divalent cationswill precipitate fast .

Part of the liquid in the reboiler or the circulation loop is taken outas stream (15) and recirculated by a control valve (16) and injected tothe rich MEG stream as (17). The correct composition of the lean MEG isobtained by keeping the temperature/pressure in the reboiler. Lean MEGproduct is taken out at the outlet (18). The lean MEG stream (18) willcontain precipitated particles of polyvalent cation salts, and these areremoved by a solids separation process before the lean MEG is sent tostorage/injected, preferably by a centrifuge or filtration unit placedin the outgoing MEG stream 18 (not included in the figure).

Alternatively, the filtration unit may also be located at stream (5) orin the circulation loop (9)-(14) to remove at least part of the solidparticles. It is also possible to have multiple filtration units atvarious locations in the process. However, it was found that it isbeneficial to have particles present to act as nucleation sites in theheaters, flash drum and reboiler. The recirculating lean MEG (15) may betaken from several locations such as directly from the reboiler (6) orfrom the outlet pipe (9). This will, however, require an extrarecirculation pump. By taking it from stream (11), the dischargepressure of the recirculation pump is used to inject the lean MEG intothe rich MEG.

The amount of lean MEG to be returned to the aqueous MEG is determinedby the temperature of the lean MEG and the temperature of the incomingaqueous MEG feed 1. By monitoring the temperatures the amount ofrecirculated hot lean MEG can be adjusted using the control valve 16, inorder to raise the temperature of the aqueous MEG feed to the desiredtemperature range of 60-120° C. Optionally, a preheater may be placedbefore the flash separator 3 in order to assist in controlling thetemperature, and for starting up the process.

1. Method for the removal of polyvalent cations from mono ethyleneglycol, comprising the steps of: a) providing a feed of aqueous monoethylene glycol comprising dissolved gas and salts of divalent cations,b) heating the aqueous mono ethylene glycol to a heated mixture, causingprecipitation of at least part of the salts and release of at least partof the dissolved gas, in particular carbon dioxide, c) separation ofreleased gas from the mono ethylene glycol, d) separation of at leastpart of the precipitated salts from the mono ethylene glycol, e)distillation of at least part of the water from the heated mixture, toyield hot dewatered mono ethylene glycol, wherein a first part of thehot dewatered mono ethylene glycol is led back to the aqueous monoethylene glycol supplied in step a) to provide at least part of the heatfor heating the aqueous mono ethylene glycol in step b).
 2. Methodaccording to claim 1, wherein in step b), the aqueous mono ethyleneglycol is heated to at least 60° C.
 3. Method according to claim 1,wherein the hot dewatered mono ethylene glycol led back to the feed hasa temperature above 100° C.
 4. Method according to claim 1, wherein theamount of hot dewatered mono ethylene glycol led back to the feed iscontrolled to obtain a warm aqueous mono ethylene glycol mixture in thetemperature range of 60° C.-120° C. in step b).
 5. Method according toclaim 1, wherein the hot cleaned mono ethylene glycol led back to thefeed comprises particles of salts of divalent cations.
 6. Methodaccording to claim 1, wherein the separation of at least part of theprecipitated salts from the mono ethylene glycol d) is performed on asecond part of the hot dewatered mono ethylene glycol that is not ledback to the feed.
 7. Method according to claim 1, wherein indistillation step e) the dewatered mono ethylene glycol is circulated ina distillation loop comprising at least one heater, wherein a first partof the dewatered mono ethylene glycol led back to the feed is taken fromthe circulating mono ethylene glycol.
 8. Method according to claim 7,wherein the dewatered mono ethylene glycol is taken from thedistillation loop after distillation and before the heater.
 9. Methodaccording to claim 7, wherein the distillation loop comprises acirculation pump, wherein the dewatered mono ethylene glycol is takenfrom the loop after the recirculation pump and before the heater. 10.Method according to claim 1, wherein the salts of polyvalent cations inthe mono ethylene glycol comprises salts derived from at least one ofthe elements selected from the group consisting of iron, calcium,barium, strontium and magnesium, or mixtures thereof.
 11. Methodaccording to claim 10, wherein the salts comprise predominantly ironsalts.
 12. Apparatus for the removal of polyvalent cations from monoethylene glycol comprising: a feed line for a feed of aqueous monoethylene glycol, connected to—a flash separator, for separating releasedgas and precipitating salts from aqueous mono ethylene glycol, whereinthe flash separator is coupled to a reboiler provided with adistillation unit for removing water from the mixture supplied thereinby heating, having an outlet to supply at least partially cleaned monoethylene glycol, wherein the outlet is provided with a loop back tosupply at least part of the hot cleaned mono ethylene glycol to the feedline for aqueous mono ethylene glycol, adapted for heating the feed inthe feed line by mixing, wherein the loop back is connected to the feedline upstream from the flash separator.
 13. Apparatus according to claim12, wherein the reboiler comprises a recirculation loop with arecirculation pump, wherein the recirculation pump is positioned to alsoprovide pressure for the loop back.
 14. Apparatus according to claim 12,wherein the feed line is provided with a preheater, wherein the loopback is introduced into the feed after the preheater.